Professional C__ - Marc Gregoire [14]
string str1 = "Hello";
string str2 = "World";
string str3 = str1 + " " + str2;
cout << "str1 is " << str1 << endl;
cout << "str2 is " << str2 << endl;
cout << "str3 is " << str3 << endl;
if (str3 == "Hello World") {
cout << "str3 is what it should be." << endl;
} else {
cout << "Hmmm . . . str3 isn't what it should be." << endl;
}
Code snippet from stringtest2\stringtest2.cpp
The preceding examples show just a few of the many features of C++ strings. Chapter 14 goes into further detail.
Nonstandard Strings
There are several reasons why many C++ programmers don’t use C++-style strings. Some programmers simply aren’t aware of the string type because it was not always part of the C++ specification. Others have discovered over the years that the C++ string doesn’t provide the behavior they need and have developed their own string type. Perhaps the most common reason is that development frameworks and operating systems tend to have their own way of representing strings, such as the CString class in Microsoft’s MFC. Often, this is for backward compatibility or legacy issues. When starting a project in C++, it is very important to decide ahead of time how your group will represent strings.
References
The pattern for most functions is that they take in zero or more parameters, do some calculations, and return a single result. Sometimes, however, that pattern is broken. You may be tempted to return two values, or you may want the function to be able to change the value of one of the variables that were passed in.
In C, the primary way to accomplish such behavior is to pass in a pointer to the variable instead of the variable itself. The only problem with this approach is that it brings the messiness of pointer syntax into what is really a simple task. In C++, there is an explicit mechanism for “pass-by-reference.” Attaching & to a type indicates that the variable is a reference. It is still used as though it was a normal variable, but behind the scenes, it is really a pointer to the original variable. Below are two implementations of an addOne() function. The first will have no effect on the variable that is passed in because it is passed by value. The second uses a reference and thus changes the original variable.
void addOne(int i)
{
i++; // Has no real effect because this is a copy of the original
}
void addOne(int& i)
{
i++; // Actually changes the original variable
}
The syntax for the call to the addOne() function with an integer reference is no different than if the function just took an integer.
int myInt = 7;
addOne(myInt);
There is a subtle difference between the two addOne() implementations. The version using pass-by-value will accept constants without a problem; for example “addOne(3);” is legal. However, doing the same with the pass-by-reference version of addOne() will result in a compiler error. This can be solved by using rvalue references, which are explained in Chapter 9.
Exceptions
C++ is a very flexible language, but not a particularly safe one. The compiler will let you write code that scribbles on random memory addresses or tries to divide by zero (computers don’t deal well with infinity). One of the language features that attempts to add a degree of safety back to the language is exceptions.
An exception is an unexpected situation. For example, if you are writing a function that retrieves a web page, several things could go wrong. The Internet host that contains the page might be down, the page might come back blank, or the connection could be lost. In many programming languages, you would handle this situation by returning a special value from the function, such as nullptr (NULL in pre-C++11) or an error code. Exceptions provide a much better mechanism for dealing with problems.
Exceptions come with some new terminology. When a piece of code detects an exceptional situation, it throws an exception. Another piece of code catches the exception and takes appropriate action. The following example shows a function, divideNumbers(), that throws an exception if the caller passes